Project Summary/Abstract X-linked hypophosphatemia (XLH) is the most common form of inheritable rickets, characterized by elevated FGF23 levels leading to low serum phosphate and impaired production of 1,25 dihydroxyvitamin D (1,25(OH)2D). Mutations in the endopeptidase PHEX are responsible for the XLH phenotype. While it is known that increases in FGF23 result from PHEX mutations, the molecular basis for this remains unknown. Pathologic mineralization of the enthesis (tendon-bone attachment site), referred to as enthesopathy, is a complication of XLH. Common sites of involvement include the Achilles and patellar entheses. Although it causes significant morbidity in patients, the molecular mechanisms responsible for the enthesopathy and impact of currently available modalities of treatment on the development of enthesopathy are poorly understood. Murine models of hypophosphatemia, including the vitamin D receptor knockout (VDR KO) and the renal type II sodium dependent phosphate co-transporter (Npt2a) KO do not have enthesopathy. In contrast, mice with hypophosphatemia and elevated serum FGF23 levels, including the Hyp mouse (murine model of XLH) and the dentin matrix protein 1 (DMP1) KO, do develop enthesopathy, suggesting FGF23 and not serum phosphate could have a pathogenic role in enthesopathy development. However, VDR KO mice do not develop rickets until 3 weeks of age, while the rickets in Npt2aKO mice reverses by 4 weeks due to increased 1,25(OH)2D levels. Since both the Hyp and DMP1 KO mice have progressive rickets from birth, the impaired skeletal mineralization may alter mechanical forces in the maturing enthesis, contributing to the development of enthesopathy in XLH and DMP1 inactivation. The studies proposed are focused on elucidating the molecular and cellular basis for this enthesopathy in a murine model of XLH (Hyp mice). Investigations proposed in Aim I will provide a molecular characterization of the abnormal cells in the entheses of Hyp mice and identify signaling pathways implicated in the development of enthesopathy. The lineage tracing studies in Aim II will identify the origin of these cells in the Hyp enthesis and the pathophysiologic basis for their aberrant differentiation. The studies proposed in Aim III will identify the relative contributions of FGF23, impaired mineralization and mechanical strain on enthesopathy development and progression. The molecular pathophysiology underlying this aberrant mineralization will also provide insight into enthesis abnormalities associated with other musculoskeletal disorders. Prior studies have focused on embryonic tendon development, thus these experiments will contribute to our understanding of normal post-natal enthesis maturation. The PI, Dr. Liu, is a physician-scientist whose long-term goal is to lead an independent basic science laboratory with a focus on musculoskeletal biology. She developed a strong interest in rare bone diseases while examining the effect of calcitonin on serum FGF23 levels in patients with XLH. She designed, executed and performed analyses for this study, which led to a first author publication in the New England Journal of Medicine. This study combined with her rigorous training in clinical endocrinology allowed Dr. Liu to recognize how clinical experiences can inform her research hypotheses. Her passion for studying the molecular pathophysiology underlying XLH led her to join the laboratory of Dr. Marie Demay, a senior investigator in skeletal biology. Dr. Liu was awarded a F32/NRSA grant to investigate how different treatment modalities, including daily 1,25(OH)2D and FGF23 blocking antibody, affect growth, mineral ion homeostasis, chondrocyte differentiation, and skeletal mineralization. A K08 award will allow Dr. Liu to take advantage of the rich mentorship available in the Harvard skeletal biology community. Her co-mentor Dr. Vicki Rosen, advisory committee, and collaborators, all renowned investigators in musculoskeletal biology, will provide her with valuable advice on experimental design and analyses, as well as career guidance. The career plan, consisting of regular meetings with mentors, course work, didactic conferences, research seminars and journal clubs is designed to enhance her research experience. Dr. Liu will develop expertise in the design, execution and interpretation of studies aimed at dissecting complex skeletal phenotypes at the molecular and cellular levels. She will refine her grant and manuscript writing skills and immerse herself in the field of skeletal and tendon biology. The carefully designed career development plan, combined with a rigorous laboratory experience will provide her with the skills necessary for her to establish her independent research program in an academic institution.